Sensing Mechanism Elucidation of a Chemosensor Based on a Metal-Organic
Framework (MOF) Selective to Explosive Aromatic Compounds
Abstract
Theoretical elucidation of the turn-off mechanism of the luminescence of
a chemosensor based on a metal-organic framework (MOF)
[Zn2(OBA)4(BYP)2] (BYP: 4,4’-bipyridine; H2OBA: 4,4’-oxybis(benzoic
acid)), selective to nitrobenzene via quantum chemical computations is
presented. The electronic structure and optical properties of Zn-MOF
were investigated through the combination of density functional theory
(DFT) and time-dependent-DFT methods. Our results indicate that the
fluorescence emission is governed by a linker (BPY) to linker (OBA)
charge transfer (LLCT) involving orbitals π-type. Next, interaction with
the analyte was analyzed, where very interesting results were obtained,
i.e. the LUMO is now composed by orbitals from nitrobenzene, which
changes the emissive state of the Zn-MOF. This fact suggests that the
LLCT process is blocked, inducing then the fluorescence quenching.
Otherwise, the Morokuma-Ziegler energy decomposition and NOCV (Natural
Orbitals for Chemical Valence) on the Zn-MOF-nitrobenzene interactions
were studied in detail, which illustrate the possible channels of charge
transfer between Zn-MOF and nitrobenzene. Finally, we believe that this
proposed methodology can be applied to different chemosensor-analyte
systems to evidence the molecular and electronic factors that govern the
sensing mechanisms.